High confinement waveguide on an electro-optic substrate

ABSTRACT

The invention relates to an optical device including a passive high confinement waveguide, such as of silicon-rich silicon nitride, on an electro-optic substrate, like lithium niobate, optically coupled to a waveguide in the electro-optic substrate. A wide range of electro-optic devices are enabled by this high confinement waveguide structure, including: directional couplers, compact tap couplers, folded electro-optic devices, electro-optic modulators including ring resonators, electro-optic gratings. Further applications enabled by the present invention include hybrid passive planar lightwave circuits (PLC) integrated with electro-optically active waveguides, using the high confinement waveguide as an intermediary waveguide to transfer optical power between the passive and active components.

TECHNICAL FIELD

The present invention relates to an optical device including a passivehigh confinement waveguide, such as of silicon-rich silicon nitride, onan electro-optic substrate, like lithium niobate, optically coupled to awaveguide in the electro-optic substrate. A wide range of electro-opticdevices are enabled by this high confinement waveguide structure,including: directional couplers, compact tap couplers, foldedelectro-optic devices, electro-optic modulators including ringresonators, electro-optic gratings. Further applications enabled by thepresent invention include hybrid passive planar lightwave circuits (PLC)integrated with electro-optically active waveguides, using the highconfinement waveguide as an intermediary waveguide to transfer opticalpower between the passive and active components.

BACKGROUND OF THE INVENTION

A high confinement waveguide for use on electro-optic substrates ishighly desirable for its ability to increase the bend radius of opticalwaveguides. This would facilitate size reduction of devices, morefunctionality and greater packing density on electro-optic chips. Afurther benefit is the creation of hybrid PLC-electro-optic chips.

Due to the small index delta between diffused waveguides, andsurrounding electro-optic substrate, such as Ti in lithium niobate, thebend radius for waveguides with acceptable loss is currently ratherlarge. This is a major limiting factor to reducing electro-optic devicesize. A higher confinement monolithically or hybrid integrated waveguidewith a greater index of refraction would enable a smaller bend radiusand smaller device features. However, if the higher index material usedto make the higher confinement waveguide is electro-optically inactive,the optical power must be transferred adiabatically between the highconfinement waveguide and lower confinement electro-optically activewaveguide. A structure is needed which can tighten the mode field of theoptical signal for passive features, like bends, while still permittingas much transmission within the electro-optic substrate as possible inother portions of the device. Monolithic or hybrid vertical integrationof low and high confinement waveguides is also desired as it will lowerthe total cost of the device by eliminating the need for butt-jointoptical transitions between substrates made of different materials,which require precision alignment.

Planar lightwave circuits (PLC) are a well developed passive opticaltechnology. Most common is a silica-on-silicon structure in whichwaveguides having a core of doped silicon dioxide (SiO₂) are depositedon an undoped silicon dioxide cladding layer, lithographically etched,and are subsequently coated with an undoped silicon dioxide uppercladding layer. The doped silica core has a slightly higher opticalindex of refraction than the cladding. Waveguides have also been made insilicon nitride SiN on a silicon substrate. The core of the siliconnitride waveguide must be much thinner and narrower than the silicawaveguide in order to allow only one guided mode to exist, because theSiN index of refraction is likely to be much higher than the doped SiO₂,making the index change, Δn, much higher.

A hybrid passive optical waveguide is described in an article by Y.Shani et al, “Integrated optic adiabatic devices on silicon” in IEEEJournal of Quantum Electronics, Vol. 27, No. 3, March 1991, pp 556-566.In that hybrid waveguide 1, as shown in FIG. 1, a stoichiometric SiNstrip (Si₃N₄) is fabricated as an inner core 2 within the doped SiO₂core 4. Most of the light is guided within the Si₃N₄ strip 2 in thishybrid waveguide 1. FIG. 2 shows an adiabatic taper 3 in the lateralwidth of Si₃N₄ 2, described by Shani above, that allows the opticalpower carried in the Si₃N₄ strip 2 to be transferred into the largermode doped SiO₂ core 4, or vice-versa, without change of mode or loss ofoptical power. FIG. 3 shows overlapping tapers 5, 7 between conventionalsilica 8 and SiN 6 waveguides also described by Shani et al. that allowadiabatic transfer of power. The cross section of the overlap region issimilar to FIG. 1 for the portion where the doped SiO₂ core 4 is widerthan the SiN strip 2.

Prior art U.S. Pat. No. 4,737,015 describes an “oxi-nitride” layer ontop of lithium niobate that is used to create a stress-inducedwaveguide. The “oxi-nitride” layer is a blend of SiO₂ and SiN. U.S. Pat.Nos. 6,670,210 and 6,864,512 also describe a waveguide containing SiO₂and SiN. It is important to note that the refractive index of the“oxi-nitride” referenced in these patents is not high enough to functionas a waveguide core, with lithium niobate as an undercladding substrate.In fact, stoichiometric SiN (Si₃N₄) has an optical index which is toolow to create a waveguide core directly over a lithium niobatesubstrate. In the prior art SiN is used to form both the core and thecladding by varying the amount of nitrogen to obtain the refractiveindex difference. Alternatively, SiO₂ is used as a cladding layer.However, this provides too much confinement for an electro-optic device.

What is needed for a high confinement waveguide on an electro-opticsubstrate, is a material having a higher refractive index than theelectro-optic substrate that can reduce the mode size of the opticalsignal. To transfer the optical signal to and from the diffusedwaveguide, the refractive index of the high confinement waveguide mustbe at least equal to or higher than the refractive index of the diffusedwaveguide. The diffused waveguide has an inhomogeneous refractive indexwith a maximum index at the top center. By contrast the high confinementwaveguide has a homogeneous refractive index and this should be higherthan an average index of the diffused waveguide. Furthermore, theoptical absorption and optical scattering losses must be low. To bepractical, the propagation loss in the SiN:Si on lithium niobate shouldbe less than 1 dB/cm.

In order to get a high enough index to create a waveguide core confinedby lithium niobate, the SiN must be silicon-rich. Silicon-rich siliconnitride waveguides are described, for example in U.S. Pat. No.6,470,130, as silicon nitrides having a ratio of greater than 3 siliconatoms to 4 nitrogen atoms per molecule. Silicon nitride compounds havingthe formula Si₃N₄ are considered stoichiometric. Silicon nitridecompounds with higher silicon content are considered silicon-richsilicon nitrides, written as SiN:Si. The silicon content of siliconnitride is controlled by changing the gas flow parameters andtemperature during deposition. As the gas parameters are changed, theindex of refraction is affected as well.

Stoichiometric SiN (Si₃N₄) waveguides are described in Shani, discussedabove, and in an article by N. Daldosso, et al., “Comparison amongvarious Si₃N₄ waveguide geometries grown within a CMOS fabrication pilotline,” IEEE Journal of Lightwave Technology, Vol 22, No 7, July 2004,pp. 1734-1740. Patches of SiN under a silica (SiO₂) core have been usedto compensate for birefringence, as described by H. H. Yaffe, et al.,“Polarization-independent silica-on-silica Mach-Zehnderinterferometers,” IEEE journal of Lightwave Technology, Vol 12, No 1,January 1994, pp. 64-67. SiN waveguides have been fabricated which haveair as a top cladding and SiO₂ as a bottom cladding, described by T.Barwicz, et al., “Fabrication of add-drop filters based onfrequency-matched microring resonators,” IEEE Journal of LightwaveTechnology, Vol 24, No 5, May 2006, pp 2207-2218. Liquid has also beenused as a top cladding with a grating in a Si₃N₄ core as described in W.C. L. Hopman, et al., “Quasi-one-dimensional photonic crystal as acompact building block for refractometric optical sensors,” IEEE Journalof Selected Topics in Quantum Electronics, Vol 11, No 1,January/February 2005, pp 11-16.

A Si₃N₄ waveguide integrated with an electro-optically active polymer isdescribed in I. Faderl, et al., “Integration of an electrooptic polymerin an integrated optic circuit on silicon,” IEEE Journal of LightwaveTechnology, Vol 13, No 10, October 1995, pp 2020-2026.

However, the prior art does not provide any teaching concerning thecreation of high confinement optical waveguides for use on anelectro-optic substrate. For the reduction of device size andflexibility of design, such a waveguide structure is highly desirable.

An object of the present invention is to provide a high confinementwaveguide for use on an electro-optic substrate and which can beoptically coupled substantially adiabatically into a waveguide withinthe electro-optic substrate.

A further object of the present invention is to provide a highconfinement waveguide on the electro-optic substrate having a small bendradius for higher device packing density.

A further object of the present invention is to provide a highconfinement optical waveguide adapted to couple light from anelectro-optic device into a passive optical device in an integratedhybrid optical device.

A further object of the present invention is to provide an electro-opticdevice including high confinement waveguides defining small devicefeatures and folded features for high packing density.

A further object of the present invention is to providepassive-electro-optic integrated devices including high confinementwaveguides providing adiabatic light transfer from the passive toelectro-optic device and vice versa.

SUMMARY OF THE INVENTION

The present invention has found that significant advantage can beobtained by creating high confinement waveguides directly on theelectro-optic substrate, such as with SiN:Si on lithium niobate coupledto diffused waveguides such as Ti, or other waveguides within theelectro-optic substrate, such as annealed proton exchange (APE)waveguides, in order to reduce the mode size for portions of thewaveguide circuit. Generally, the high confinement waveguide will not bethe only waveguide in the electro-optic device, because only the tail ofthe optical mode is transmitted through the electro-optic material in ahigh confinement waveguide. Consequently, the electro-optic effect islimited. This is acceptable for certain applications. It is preferred tocombine the high confinement waveguide for small bend radius areas withTi diffused waveguides for straight sections and for better mode sizematching to optical fiber. Alternatively, a hybrid waveguide in which ahigh confinement core is coincident with a diffused waveguide of similarrefractive indices can be created to provide optical transmission in theelectro-optic substrate with a smaller mode size.

Accordingly, the present invention relates to a high confinementwaveguide comprising: an electro-optic substrate having a refractiveindex n_(s); an optical waveguide within the electro-optic substratehaving a refractive index n_(w) greater than n_(s); a high confinementwaveguide on the electro-optic substrate optically coupled to theoptical waveguide, the high confinement waveguide having a refractiveindex n_(c) greater than n_(s) such that the electro-optic substrateinduces total internal refraction within the high confinement waveguide,and a refractive index n_(c) greater than n_(w) such that most of theoptical power will couple from the optical waveguide to the highconfinement waveguide when the high confinement waveguide is in contactwith the optical waveguide.

Another aspect of the present invention relates to an electro-opticdevice comprising: an electro-optic substrate having a refractive indexn_(s); at least one optical waveguide within the electro-optic substratefor transmitting an optical signal through the device for electricalmodulation; at least one high confinement waveguide having a refractiveindex n_(c) greater than n_(s) optically coupled to the at least oneoptical waveguide through at least one taper for adiabatic transfer ofthe optical signal.

Another feature of the present invention provides an integrated opticaldevice comprising: an electro-optic element disposed on an electro-opticsubstrate having a refractive index n_(s); a passive optical element;and an optical waveguide circuit through the electro-optic element andthe passive optical element, wherein the optical waveguide circuitincludes a high confinement waveguide on the electro-optic elementhaving a refractive index n_(c) higher than n_(s) and a high confinementwaveguide on the passive optical element having a refractive index n_(p)optically coupled to the high confinement waveguide of the electro-opticelement.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 is a cross-section of a prior art hybrid SiN silica waveguide;

FIG. 2 is a top view of the prior art waveguide of FIG. 1 illustratingan optical adiabatic taper;

FIG. 3 is a top view of a prior art transition of overlapping tapersbetween conventional silica and SiN waveguides for adiabatic transfer ofoptical power;

FIG. 4 is a cross-section of two high confinement waveguides inaccordance with the present invention;

FIG. 5 is a cross-section of two alternate high confinement waveguidesin accordance with the present invention;

FIG. 6 is a cross-section of a high confinement waveguide in combinationwith a ridge formed substrate as often used in modulator structures;

FIG. 7 is a cross-section of several high confinement waveguidespartially or completely buried in the electro-optic substrate;

FIG. 8 is an isometric view of a high confinement waveguide formed in atrench having a tapered depth in the electro-optic substrate enabling agradual transition from low to high confinement;

FIG. 9 is a top view of an adiabatic taper for use in the presentinvention similar to the taper of FIG. 2;

FIG. 10 is a top view of a transition made of overlapping tapers for usein the present invention similar to FIG. 3;

FIG. 11 is a cross-section of an optical coupler in accordance with thepresent invention for horizontal evanescent coupling;

FIG. 12 is a cross-section of three examples of optical couplers forvertical evanescent coupling;

FIG. 13 is a top view of the coupler of FIG. 11;

FIG. 14 is a top view of one of the couplers of FIG. 12;

FIGS. 15A and 15B are color-enhanced graphic plots of the opticalE-field of the coupler of FIGS. 11 and 13 as 3D BPM simulationscalculated at several cross-sections from the first adiabatic taper tothe center FIG. 15A, and from the center to the second adiabatic taperFIG. 15B;

FIG. 16 is a top view of an abrupt bend in the high confinementwaveguide in accordance with the present invention;

FIG. 17 is a top view of an alternate structure of an abrupt bend in ahigh confinement waveguide;

FIG. 18 is a top view of a further alternate structure of an abrupt bendin a high confinement waveguide;

FIG. 19 is a schematic view of an electro-optic chip including a tapcoupler with a sharp bend for guiding tapped light to the edge of theelectro-optic chip;

FIG. 20 is a schematic illustration of a folded Mach Zehnder modulatorusing sharp bends in high confinement waveguides to fold the modulator;

FIG. 21 is a cross-section of a SiN on LN waveguide with a buffer layerand electrode on top to provide modulation;

FIG. 22 is a cross-section of an alternative SiN on LN waveguide asshown in FIG. 21;

FIG. 23 is a top schematic view of an electro-optically controlled ringresonator;

FIG. 23A is a cross-section of the ring resonator of FIG. 23 takenthrough line 12-12;

FIG. 24 is a schematic view of an alternative ring resonatorconfiguration;

FIG. 25 is a schematic view of a further alternative ring resonator inaccordance with the present invention;

FIG. 26 is a schematic longitudinal section of a SiN on LN waveguidewith a grating etched into the LN in accordance with a furtherembodiment of the present invention;

FIG. 27 is a schematic cross-section of a hybrid integration of a silicaon silicon device to be optically coupled by a high confinementwaveguide to an electro-optic chip in a flip-chip orientation;

FIG. 28 is a schematic cross-section of an alternative structure of thehybrid integration of FIG. 27;

FIG. 29 is a schematic cross-section of the device of FIG. 27 with thehigh confinement waveguide optically coupling the optical andelectro-optical devices;

FIG. 30 is a schematic cross-section of the device of FIG. 28 opticallycoupled together;

FIG. 31 is a top view of the two devices of FIG. 27 assembled togetherand schematically illustrating the optically coupled waveguides acrossthe hybrid device.

DETAILED DESCRIPTION

FIGS. 4-31 describe various embodiments of the invention, which consistof a high confinement waveguide, like SiN:Si, formed on top of anelectro-optic material, like lithium niobate (LN). FIGS. 4-6 show thecross sections of different types of SiN:Si-on-LN waveguides. The SiNstrip 10 is made to be rich in Si, increasing its optical index ofrefraction to be slightly higher than the optical index of LN 20. A highconfinement waveguide must have an index change relative to theelectro-optic substrate that is significantly larger than that createdin the diffused waveguide. The maximum refractive index in a diffusedwaveguide in lithium niobate is typically 0.01 to 0.02 higher than thesubstrate index or said another way, the index change forming thewaveguide is less than 1% of the substrate index. Furthermore, theaverage index change in most of the waveguide is less than the maximumindex change as a result of creating a waveguide with a diffusionprocess, hence the average index change may be less than 0.5%. A highconfinement waveguides made with a SiN strip having an index ofrefraction 0.05 or more above that of the substrate has an average indexchange that is at least 2% of the substrate index, which is severaltimes larger than that created within the diffused waveguide. Generally,the index change of the high confinement waveguide should be at least0.02 and as high as 0.2. More preferably the index change is 0.02-0.1.And most preferably the index change is 0.05.

A diffused waveguide 12, such as of Ti, or other materials (nickel,magnesium-oxide, zinc oxide, rare earth, etc.), is present in theelectro-optic substrate 20 at least to couple light into or out of thehigh confinement waveguide 10, and often for more substantial overlap.The titanium is diffused into the LN at high temperatures to form theTi-diffused waveguide. As mentioned above, the Ti waveguide 12 bringsthe optical signal into the electrically active region of the substrate20. When the light is guided by the high confinement waveguide 10, verylittle of the optical signal is exposed to the electrical field. Also,the larger mode size of the diffused waveguide 12 provides a bettermatch for coupling into optical fiber. Normally, most of the opticalpower is carried in the SiN:Si strip 10, regardless whether the SiN:Sistrip 10 is on undoped LN 20 as shown in FIG. 5, or if the SiN:Si strip10 is on top of a Ti-diffused LN waveguide 12 as shown in FIG. 4. FIG. 4shows an SiN:Si-on-LN waveguide 10 with and without an upper cladding 14consisting of doped or undoped SiO₂, a material found in many LNmodulators. Dopants may be introduced to tailor either electrical and/oroptical properties of the SiO₂. The upper cladding 14 can function toprotect the SiN:Si-on-LN waveguide 10, and can also include a bleedlayer like TaSiN on top of it, to bleed off pyroelectric charge from theLN 20 and/or act as encapsulant, to keep out moisture. FIG. 5 showsSiN:Si-on-LN waveguides 10 that consist only of the SiN:Si strip 10 onLN 20, without the Ti-diffused waveguide.

FIG. 6 shows that the SiN:Si-on-LN waveguide 10 can be used incombination with etched slots 22 that form ridges in the substrate 20,which are often used to improve modulation efficiency in LN modulators.This structure allows for modulation along a tight bend. Note that theSiN:Si material is electro-optically inactive, however, the tail of themode is within the electro-optically active substrate. Most likely, themodulation within a SiN:Si-on-LN waveguide will be much weaker thanwithin a LN waveguide, as most of the optical power is confined to theSiN. The ridge structure improves modulation efficiency, to help offsetsome of the loss of modulation efficiency due to the electro-opticinactivity of the SiN.

FIGS. 7 and 8 show how the SiN:Si strip 10 may be partially orcompletely buried within the LN substrate 20. Burying the SiN:Si strip10 reduces the lateral index change, reducing confinement in the stripto reduce optical loss in the transition from a larger mode Ti-diffusedwaveguide 12 to the hybrid SiN:Si plus Ti-diffused waveguide. Buryingthe SiN:Si strip 10 also increases the amount of mode tail in theelectro-optic substrate, thereby improving modulation efficiency.Depositing the SiN:Si material 10 into a wedge shaped trench 24 causesthe volume of SiN:Si contained within the lithium niobate 20 to begradually reduced as the material fills the trench 24 conformally andmore of the strip 10 is surrounded by air, as shown in FIG. 8. Thisincreases confinement in a more gradual manner, thereby reducing opticalloss from lower confinement to high confinement power transfer.Alternatively the confinement can be modified gradually by usingperiodic segmentation of the high confinement waveguide 10 alternatinghigh confinement material in a progressing duty cycle with a claddingmaterial having an index of refraction closer to the index of the highconfinement waveguide than air the transition medium. For example, thecladding in the region with periodic segmentation could bestoichiometric SiN, which has an index close to that of LN and SiN:Si.The cladding could cover all of the high confinement waveguide.Alternatively, the SiN cladding could also be patterned, to put it onlyin locations having periodic segmentation. Tapering the width of theupper cladding from wider to less than that of the SiN:Si strip 10 wouldreduce the scattering loss at the transition from regions with uppercladding to those without.

Shani et al. provide a useful definition for “adiabatic.” As used inthis application, it means that the occupations of the optical modes ofthe system do not change as the waveguide structure changes. If thefundamental mode is initially excited, all the power stays in thefundamental mode as the waveguide structure and hence the mode shapechanges. No power is coupled to other modes or radiated into thecontinuum. FIG. 9 shows an adiabatic taper 11 similar to prior art FIG.2 in layout. The taper 11 transfers power vertically from diffusedwaveguide 12 in LN 20 to the SiN:Si strip 10 on LN 20. The shape andsize of the mode changes dramatically as the power is shifted from onewaveguide to another. However, since the optical signal is attracted tothe higher index contrast, very little optical power is lost in thetransfer. FIG. 10 shows another taper design 13 similar in layout tothat in prior art FIG. 3. In FIG. 10, the power is adiabaticallytransferred vertically from the SiN:Si strip 10 to the diffusedwaveguide 12 in LN 20, or vice-versa, depending on which direction thelight is propagating. The cross section of the tapers in FIGS. 9 and 10are similar to that shown in FIG. 4, for the sections where the diffusedwaveguide width is wider than the SiN:Si strip width.

Other waveguide structures can be designed with the high confinementwaveguides. FIGS. 11 and 12 show the cross sections of SiN:Si-on-LNoptical couplers with either horizontal or vertical evanescent coupling.The horizontal coupler 30 is simpler to fabricate, though, requiresprecise control of lateral dimensions. The vertical coupler 32, 34, 36is more complex, but requires precise control of vertical dimensions,which are sometimes easier to control with great precision. Thestructure in FIG. 11 includes a Ti-diffused waveguide 12 to facilitatecoupling between the optical modes in the SiN:Si waveguides 10. Themodes have more overlap within the Ti-diffused waveguide 12 than they doin the region above the LN, where there is a huge index change.

FIG. 12 shows three versions of a vertical coupler using a SiN:Si-on-LNwaveguide. In vertical coupler 32, the lower SiN:Si-on-LN waveguide 10has a Ti-diffused waveguide 12 below it, though, the coupling occurswithin the SiO₂ buffer layer 16, where the mode tails of the two SiN:Siwaveguides 10 overlap. The vertical coupler 34 has one SiN:Si strip 10directly on top of the LN 20 and a second one directly over top of itwith the buffer layer 16 in between. There is no Ti-diffused waveguidein coupler 34. Vertical coupler 36 consists of a Ti-diffused waveguide12 and an SiN:Si strip 10 on top of the buffer layer 16. The amount ofcoupling may be low in coupler 36, as the mode indices of the twowaveguides are probably different enough to spoil the coupling. Thecoupler 36 can function well as a tap coupler, where the desired amountof coupled light is small. Note also that the optical index ofrefraction of the LN substrate may change more with wavelength than theoptical index of the SiN:Si material, causing the amount of coupling tobe wavelength dependent.

The variation in LN substrate index with wavelength can be used tocompensate for the variation in coupling caused by mode overlap. Forexample, the tail of the LN mode in coupler 36 in the SiO₂ buffer layermay get larger with increasing wavelength, leading to more coupling. Ifthe optical index of the SiN:Si is slightly larger, but close to that ofthe Ti-indiffused waveguide, the amount of optical phase mismatchbetween the modes of the two waveguides will actually increase withwavelength, possibly offsetting the increased coupling due to more modeoverlap. The optical phase mismatch increases due to the wavelengthdependence of the index of the LN substrate which has a refractive indexwhich decreases with wavelength. If the SiN:Si waveguide propagationconstant is roughly constant with wavelength, then the difference inoptical propagation constants will increase with wavelength. Thecompeting effects of mode overlap and optical phase mismatch can be usedto create a coupler that has a coupling ratio that is roughly constantover some wavelength range.

FIGS. 13 and 14 show top views of the couplers described in FIGS. 11 and12. In FIG. 13, the Ti-diffused waveguides actually merge in thecoupling region. Line 11-11 shows the section shown in FIG. 11 throughthe center of the coupling region. Adiabatic tapers 11 and 13 transferpower from the diffused waveguide 12 into the SiN:Si strip 10 and backagain. Within the coupling region, most of the light is within theSiN:Si strip 10 on top of the straight waveguide 10, however, some lightis carried within the diffused waveguide 12, and there is some overlapof the optical modes of the two hybrid waveguides within the mergeddiffused waveguide region. The overlap is large enough to permit someoptical power to be transferred to the tap waveguide 10′ that has thesharp waveguide bends. The sharp waveguide bends permit the tapped lightto be redirected away from the straight waveguide in a short distance,making it possible to integrate the coupler in a short section ofstraight waveguide. FIG. 14 shows a top view of coupler 32 of FIG. 12.Section line 12-12 shows the cross-section through the center of thecoupling region as shown in FIG. 12. An adiabatic taper 11 transfersmuch of the optical power into the first SiN:Si strip 10 on top of theLN. Evanescent coupling to the second SiN:Si strip 10′ on top of thebuffer layer 16 transfers some optical power to the second SiN:Si strip10′. The tapped power is directed away from the bottom optical waveguide10 with a tight bend in the second SiN:Si strip 10′.

FIGS. 15A and 15B show color-enhanced plots from a 3D BPM simulation ofthe horizontal coupler shown in FIGS. 11 and 13. Plots of the crosssection of the optical E-field along the coupler are stacked on top ofone another. The first stack of plots in FIG. 15A shows the firstadiabatic taper 11 and coupled section, including the bends for the taparm of the coupler. About 3% of the optical power is coupled into thetap arm. The second stack of plots in 15B shows the second adiabatictaper 13, where light is transferred from being mostly in the SiN:Sistrip back into the diffused waveguide. Note the dramatic change in modesize in shape resulting from transfer of optical power between theconventional Ti-diffused and hybrid SiN:Si-on-LN waveguide. The smallermode size of the hybrid SiN:Si-on-LN waveguide makes tighter waveguidebends possible, which greatly reduces the device length needed for a tapcoupler. In fact, most likely the tap coupler could be integrated intoan LN modulator without adding any device length.

FIGS. 16, 17 and 18 show top views of abrupt bends that have a beveledcorner 40 that acts like a mirror. The SiN:Si strip 10 has a verticalsidewall at the beveled corner 40, fabricated by either etching orlift-off. The vertical sidewall is necessary for low optical loss at theabrupt bend. The large index contrast of the high confinement waveguidemakes this possible without additional metallization of the reflectorstructure. FIGS. 16 and 17 show 90° abrupt bends with or without aTi-diffused waveguide 12 underneath the SiN:Si strip 10, respectively.FIG. 18 shows an abrupt bend with a Ti-diffused waveguide, where thebend angle is less than 90°. Multiple abrupt bends having individualbend angles less than 90°, but an accumulated bend angle of 90°, mighthave lower optical loss from radiated or scattered light than one single90° bend.

There are multiple applications of SiN:Si-on-LN waveguides. TheSiN:Si-on-LN or other high confinement waveguide facilitates tighteroptical bends than are possible with a diffused waveguide likeTi-diffused. Other materials can be used in place of SiN:Si. The maincriteria is that (1) the optical index must be slightly higher than theLN substrate optical index, and also larger than the optical index ofthe Ti-diffused waveguide, and (2) the optical absorption and opticalscattering losses must be low. To be practical, the propagation loss inthe SiN:Si-on-LN waveguide must be less than 1 dB/cm, and losses in theadiabatic tapers, where light is transferred from one type of waveguideto another, must be less than a few tenths of a dB.

FIG. 19 shows a SiN:Si-on-LN coupler 38 similar to the ones described inFIGS. 11-15 as integrated into an electro-optic Mach-Zehnder device 50.The higher confinement allows one arm 10′ of the coupler 38 to bend 90°to guide the tapped light to the side of the LN chip 20, where thetapped light is directed to a photodetector 52 mounted on the shelf 54of the package holding the chip.

FIG. 20 shows the optical waveguides 62 of a folded modulator 60 using90° SiN:Si waveguide bends 64 to fold the MZ. The bends may use one ormore abrupt bends with a beveled corner as shown in FIGS. 16-18 or anarc with a small radius. Tap coupler 38 is illustrated at the modulatoroutput 66. By folding the modulator 60, a much longer interaction lengthcan be achieved without requiring a large dimension in the chip.

FIGS. 21 and 22 show a SiN-on-LN waveguide 10 in cross-section with abuffer layer 16 and electrodes 68, 70 on top to provide modulation. Thestructure shown in FIG. 21 has a Ti-diffused waveguide 12 underneath theSiN strip 10, while the one in FIG. 22 does not. Note that only theTi-diffused waveguide and LN substrate are electro-optically active. TheSiN:Si strips are electro-optically inactive.

The structures in FIGS. 21 and 22 make it possible to modulate the lightalong a tight bend, thereby increasing the level of integration.However, there is a huge trade-off between how tight the bend can be andthe strength of modulation. The greater the confinement and more of theoptical mode that resides within the SiN:Si, the tighter the bend can beat expense of modulation efficiency. Most likely modulation efficiencywill be much lower than for a conventional LN waveguide, however, it maybe adequate for certain applications that only require weak modulation,for example the fine tuning of a coupler coupling ratio. The modulationefficiency can also be improved by resonant structures discussed later.

FIG. 23 shows another application of SiN:Si-on-LN waveguides, as anelectro-optically controlled ring resonator shown schematically as 80.Ring resonators are being used to enhance modulation of light in Siliconwaveguides as discussed in an article by B. Jalali, et al., “Siliconphotonics,” IEEE Microwave Magazine, June 2006, pp. 58-68. Light iscoupled into the ring 82, making many passes around the ring. Each timearound the ring, a small portion of the light is coupled out into thestraight waveguide 84. Maximum transmission occurs when all the portionsof light traveling in and out of the ring 82 are in phase with eachother and in phase with the portion of light traveling through thestraight waveguide 84. As illustrated the ring resonator 80 includes ahybrid waveguide of SiN 10 over Ti 12 for both the straight waveguide 84and the ring 82. FIG. 23A shows a cross-section of the structure in FIG.23. The optical index of the Ti-diffused waveguide and LN substrate areboth affected by the applied field, allowing the optical mode index ofthe hybrid waveguide to be weakly tuned. The resonance wavelength of thering resonator is shifted with the applied voltage on the signalelectrode 86. Ground electrodes 88 surround the signal electrode 86. Thering resonator 80 functions as a tunable filter or modulator. A ringresonator can be formed without the Ti-diffused waveguide 12, however,the Ti-diffused waveguide enhances modulation efficiency by increasingthe size of the tail of the mode in the electro-optically active LN.

One problem with high speed operation of ring resonators used tomodulate light is the chirp in wavelength that occurs with the change inoptical intensity. The push-pull configuration shown as ring resonator90 in FIG. 24 helps to reduce the chirp due to the balanced nature ofthe Mach-Zehnder (MZ) Interferometer 92 having a ring 94 on each arm 96.The chirp produced by the two ring resonators 94 are approximately equalin magnitude but opposite in sign, resulting in little residual phasechange for the light exiting the MZ 92 at waveguide 98. Note that morethan one ring 94 can be serially integrated within each arm 96 of the MZ92. In such an arrangement, all of the rings in one arm of the MZ wouldbe biased to the same transmission point, and driven with +Vmod1, whileall of the rings in the other arm of the MZ would be all biased to thesame transmission point, and driven with −Vmod1.

FIG. 25 shows a ring resonator topology 90′ similar to that in FIG. 24,however, the SiN:Si-on-LN waveguides are only used at (1) the corners 95of an oval shaped ring 94 and at (2) the couplers 97 used to couplelight in and out of the ring 94. Adiabatic tapers, such as shown in FIG.10, before and after each section of SiN:Si-on-LN allow light to betransferred back and forth between the high confinement SiN:Si-on-LNwaveguides and highly electro-optically active Ti-diffused waveguides.The transfer of power between the two types of waveguides improvesmodulation electro-optic efficiency at the expense of higher opticalloss in the ring, which results in lower finesse and reduced extinctionratio.

FIG. 26 shows an SiN:Si-on-LN waveguide 10 over a Ti-diffused waveguide12 with a grating 100 etched into the LN 20. As with the ring resonator80, 90, the wavelength of maximum or minimum transmission through thegrating 100 can be tuned with an applied field from an electrode.

FIGS. 27-30 show how SiN:Si waveguides assist in hybrid integration ofSilica-on-Silicon waveguide technology with lithium niobate modulatortechnology. There has been interest in integrating passive opticalcircuits with electro-optic devices for quite a few years. Recentlyinterest in high speed switching has motivated development of a MachZehnder interferometer based switch architecture in which opticalcircuits including directional couplers are realized in passive PLC andonly phase shifters are integrated in lithium niobate technology, asdescribed by K. Suzuki et al., “High-speed optical 1×4 switch based ongeneralized Mach-Zehnder interferometer with hybrid configuration ofsilica-based PLC and lithium niobate phase-shifter array,” IEEEPhotonics Technology Letters, Vol 19, No 9, May 1, 2007, pp 674-676. Amajor problem addressed by this design is insertion loss (IL). Suzuki etal. reduce the IL by reducing the number of coupling points. SignificantIL is still experienced at the interface between the silica and LNwaveguides, which are butt coupled with an anti-reflective coatingbetween them. Alignment at these points is critical. Hybrid integrationof passive silicon waveguides with active III-V quantum layer structureshas also been published by A. Fang, et al., “Hybrid silicon evanescentdevice platform,” IEEE LEOS Newsletter, April 2007, pp 4-11. However,the optical power is never completely transferred to the active III-Vmaterial. Some or most of the optical power resides within the siliconwaveguide structure.

The hybrid integration in accordance with the present invention takesadvantage of the attraction of the optical signal to the large indexcontrast of the high confinement waveguide. Vertical coupling throughvertically-stacked adiabatic tapers as shown in FIG. 10 couple a highconfinement waveguide on a LN substrate to a high confinement waveguideon a PLC. The tolerance for horizontal alignment is significantly morerelaxed than the butt coupling of the prior art. FIGS. 27 and 28 showside views of Silica-on-Silicon and LN devices apart while FIGS. 29 and30, respectively, show the structures assembled together.

As seen in FIG. 27, a silica-on silicon PLC 110 comprises a siliconsubstrate 112, an SiO₂ lower cladding layer 114, a doped SiO₂ waveguidecore 116 and an upper cladding layer 118. In addition, a highconfinement SiN:Si waveguide 120 is optically coupled to the SiO₂ core116 with an adiabatic taper. It is not essential that the highconfinement waveguide 120 has an index of refraction n_(p) that is equalto n_(c) of high confinement waveguide 138. The confinement is relativeto the substrate 112. Waveguide 120 is tapered at the opposite end toforce adiabatic transfer to high confinement waveguide 138 on theelectro-optic device 130. For assembly alignment, a spacer 121 of SiN:Siis deposited simultaneously with the high confinement waveguide 120. Theelectro-optic device 130 comprises a LN substrate 132 including aTi-diffused waveguide 134 and an etched slot 136 for receiving theoptical waveguide portion of the PLC 110. A high confinement waveguide138 of SiN:Si is optically coupled with an adiabatic taper to transferoptical power vertically to the Ti-diffused waveguide 134. At itsopposite end waveguide 138 also has a taper for adiabatic transfer tothe taper of waveguide 120 (seen more clearly in FIG. 31). As in the PLC110, a SiN spacer 139 is deposited simultaneously with the highconfinement waveguide 138 to preserve alignment.

As shown in FIG. 28, the waveguide structure of the PLC 110 is slightlydifferent from FIG. 27. FIG. 27 shows the SiN:Si strip waveguide 120below the (Ge) doped SiO₂ waveguide core 116 before being inverted inthis flip-chip orientation. In this case, the doped waveguide core 116is deposited after the SiN:Si strip 120. Alternatively, the doped SiO₂core layer 116 can be deposited before the SiN:Si 120 as shown in FIG.28. The SiN:Si layer forms a waveguide 120 on top of the doped SiO₂ core116 in the unflipped orientation. The SiN:Si waveguide 120, itself, canbe used as an etch stop when removing the SiO₂ upper cladding 114 toexpose the evanescent tail of the mode in either case. An additionalthin SiO₂ layer 115 (shown in FIG. 28) can be deposited after exposingthe SiN:Si 120 to control the mode coupling interaction. As shown inFIG. 28, high confinement waveguides 120 and 138 form a verticaldirectional coupler as shown in FIG. 12. Whereas the high confinementwaveguides 120 and 138 as shown in FIG. 27 are optically coupled asadiabatic tapers as shown in FIG. 31.

FIG. 31 shows a top view of the two devices 110, 130 togetherschematically illustrating the optically coupled waveguides. Adiabatictapers 135 couple light from a Ti-indiffused LN waveguide 134 into aSiN:Si-on-LN waveguide 138. Another set of adiabatic tapers 125transfers the light into an SiN:Si waveguide 120 integrated on aflipped-chip Silica-on-Silicon device 110. Finally, another set oftapers 117 transfer the light into a doped SiO₂ (silica) core 116. TheSilica-on-Silicon waveguide 116 steers the light through a 180°turn-a-round, after which the process is reversed to eventually returnthe light to a second Ti-indiffused LN waveguide 134. Note that the SiO₂cladding 118 and air isolate the doped SiO₂ (silica) core 116 from theLN substrate 132.

There are a wide variety of hybrid devices and designs possible withintegrated Silica-on-Silicon and LN waveguide technologies. Theselection of which device is flipped can be reversed, i.e., the LNdevice can be flipped-chip mounted onto a Silica-on-Silicon device.Other device functions are possible, as well. For example, aftermodulation in an LN device, polarization rotation, beam combining, andcoupling into an output fiber can be accomplished in a Silica-on-Silicondevice. In fact, all passive functions can be accomplished in aSilica-on-Silicon device, while all high-speed modulation functions canbe performed in the LN device.

The hybrid technology can even assist in manufacture of LN devices. Apassive optical probe head consisting a Silica-on-Silicon deviceattached to an optical fiber can couple light in or out of aTi-indiffused LN waveguide via the SiN:Si waveguides, allowing forwafer-level optical testing without the need for dicing and polishingthe endfaces of the LN device. The SiN:Si waveguides (one in the opticalhead, one on the LN) are temporarily brought in optical contact witheach other while testing a particular device on the LN wafer. Theoptical probe can be moved from device-to-device on the chip. Theadiabatic tapers allow for much increased lateral alignment tolerance,making alignment simpler than the traditional butt coupling of fibers tothe endface of an LN waveguide.

Another compliant material, with optical index similar to that ofSiN:Si, can be used as “optical glue” between SiN:Si waveguides. Thiscan be used for improving coupling between the Silica-on-Silicon deviceand LN device, for the case where either or both of the substrates arenot flat enough to allow for intimate contact between all of the SiN:Siwaveguides across the entire surface of the device. An example of asuitable optical glue consists of particles of high index material,e.g., TiO₂ or a high-index semiconductor, such as silicon or InP,suspended in an epoxy resin.

Note that other LN waveguide technologies are compatible withSiN:Si-on-LN waveguides, for example, Annealed Proton Exchanged (APE)waveguides in place of diffused waveguides. Other materials can be usedin place of SiN:Si, assuming their optical index and optical propagationloss satisfy the requirements discussed earlier. Other passivewaveguides can be used in place of Silica-on-Silicon waveguides, forexample ion-exchanged glass waveguides.

1. A high confinement waveguide comprising: an electro-optic substratehaving a refractive index n_(s); an optical waveguide within theelectro-optic substrate having a refractive index n_(w) greater thann_(s); a high confinement waveguide on the electro-optic substrateoptically coupled to the optical waveguide, the high confinementwaveguide having a refractive index n_(c) greater than n_(s) such thatthe electro-optic substrate induces total internal refraction within thehigh confinement waveguide, and a refractive index n_(c) greater thann_(w) such that most of the optical power will couple from the opticalwaveguide to the high confinement waveguide when the high confinementwaveguide is in contact with the optical waveguide.
 2. A highconfinement waveguide as defined in claim 1, wherein the index changebetween n_(s) and n_(c) is at least 0.02 and as high as 0.2.
 3. A highconfinement waveguide as defined in claim 2, wherein the index changebetween n_(s) and n_(c) is in the range of 0.02-0.1.
 4. A highconfinement waveguide as defined in claim 2, wherein the index changebetween n_(s) and n_(c) is 0.05.
 5. A high confinement waveguide asdefined in claim 1, wherein the electro-optic substrate is lithiumniobate and the high confinement waveguide is silicon-rich siliconnitride.
 6. A high confinement waveguide as defined in claim 1 furtherincluding an upper cladding layer encasing the high confinementwaveguide on the electro-optic substrate.
 7. A high confinementwaveguide as defined in claim 1, wherein the high confinement waveguideis terminated by a sharp or blunted taper to force adiabatic transfer ofan optical signal into an adjacent waveguide.
 8. A high confinementwaveguide as defined in claim 7, wherein the sharp or blunted taper ofthe high confinement waveguide overlaps a tapered end of the opticalwaveguide to force adiabatic transfer of an optical signal between thehigh confinement waveguide and the optical waveguide in eitherdirection.
 9. A high confinement waveguide as defined in claim 1,wherein, the high confinement waveguide includes an abrupt bend havingbeveled corner comprising a vertical sidewall for reflecting an opticalsignal around the abrupt bend.
 10. A high confinement waveguide asdefined in claim 1, wherein the high confinement waveguide is at leastpartially disposed in a trench in the surface of the electro-opticsubstrate.
 11. A high confinement waveguide as defined in claim 10,wherein the high confinement waveguide is partially disposed in a trenchhaving a taper for providing a gradual transition from lower to higherconfinement of the optical signal.
 12. A high confinement waveguide asdefined in claim 1, wherein the high confinement waveguide and theoptical waveguide are coincident to form a hybrid waveguide for singlemode transmission.
 13. A high confinement waveguide as defined in claim12, wherein the electro-optic substrate has a surface profile defining aridge, and the optical waveguide is formed in the ridge with the highconfinement waveguide coincident above the optical waveguide on theridge.
 14. A high confinement waveguide as defined in claim 12, whereinthe optical waveguide comprises diffused titanium and the highconfinement waveguide comprises silicon-rich silicon nitride.
 15. Anelectro-optic device comprising: an electro-optic substrate having arefractive index n_(s); at least one optical waveguide within theelectro-optic substrate for transmitting an optical signal through thedevice for electrically-induced modulation; at least one highconfinement waveguide having a refractive index n_(c) greater than n_(s)optically coupled to the at least one optical waveguide through at leastone taper for adiabatic transfer of the optical signal.
 16. Anelectro-optic device as defined in claim 15, wherein the devicecomprises a directional coupler having a first waveguide and a secondwaveguide disposed for evanescent coupling between them, and at leastone of the first and second waveguides is a high confinement waveguide.17. An electro-optic device as defined in claim 16, wherein the firstand second waveguides are high confinement waveguides verticallyseparated by a buffer layer between them.
 18. An electro-optic device asdefined in claim 17, wherein the first high confinement waveguide is ahybrid waveguide coincident with an optical waveguide in theelectro-optic substrate.
 19. An electro-optic device as defined in claim16, wherein the first and second waveguides each comprise a hybridwaveguide having a high confinement waveguide coincident with an opticalwaveguide in the electro-optic substrate, and the first and secondwaveguides are disposed in a horizontally separated side-by-sideconfiguration.
 20. An electro-optic device as defined in claim 19,wherein the optical waveguides of the first and second hybrid waveguidesoverlap in a coupling region.
 21. An electro-optic device as defined inclaim 16, wherein the directional coupler comprises a tap coupler andthe first waveguide is an optical waveguide and the second waveguide isa high confinement waveguide having a small bend radius.
 22. Anelectro-optic device as defined in claim 15, wherein the devicecomprises a Mach-Zehnder interferometer comprising an input opticalwaveguide, a first optical splitter, a first arm and a second arm, anoptical combiner and an output optical waveguide, and wherein the firstarm and second arm of the Mach-Zehnder interferometer comprise opticalwaveguides including a plurality of small radius bends comprising highconfinement waveguides optically coupled with adiabatic tapers to theoptical waveguides, in order to reduce a length of the electro-opticdevice.
 23. An optical device as defined in claim 15, wherein the devicecomprises a ring resonator including a straight waveguide having a highconfinement waveguide coincident with an optical waveguide in theelectro-optic substrate, and a ring waveguide comprising a highconfinement waveguide coincident with an optical waveguide in theelectro-optic substrate disposed for evanescent coupling between them.24. An electro-optic device as defined in claim 15, wherein the devicecomprises a Mach-Zehnder interferometer comprising an input opticalwaveguide, a first optical splitter, a first arm and a second arm, anoptical combiner and an output optical waveguide, and wherein the firstarm and second arm of the Mach-Zehnder interferometer comprise opticalwaveguides including at least one ring resonator optically coupled tothe first arm, and at least one ring resonator of equal number opticallycoupled to the second arm, and wherein the at least one ring resonatorcoupled to each arm comprises a high confinement waveguide disposed suchthat the at least one ring resonator coupled to the first arm and the atleast one ring resonator coupled to the second arm have equal biasvoltage and opposite signs.
 25. An electro-optic device as defined inclaim 23, wherein the ring resonators comprise hybrid waveguides of highconfinement waveguides coincident with optical waveguides in theelectro-optic substrate.
 26. An electro-optic device as defined in claim23, wherein the ring resonators comprise optical waveguides including aplurality of small radius bends comprising high confinement waveguidesoptically coupled with adiabatic tapers to the optical waveguides. 27.An electro-optic device as defined in claim 15, wherein the devicecomprises a grating formed as a periodic structure along the length ofthe high confinement waveguide optically coupled to the opticalwaveguide in the electro-optic substrate.
 28. An integrated opticaldevice comprising: an electro-optic element disposed on an electro-opticsubstrate having a refractive index n_(s); a passive optical element;and an optical waveguide circuit through the electro-optic element andthe passive optical element, wherein the optical waveguide circuitincludes a high confinement waveguide on the electro-optic elementhaving a refractive index n_(c) higher than n_(s) and a high confinementwaveguide on the passive optical element having a refractive index n_(p)optically coupled to the high confinement waveguide of the electro-opticelement.
 29. An integrated optical device as defined in claim 28,wherein the high confinement waveguide of the electro-optic element isoptically coupled to the high confinement waveguide of the passiveoptical element through adiabatic tapers.
 30. An integrated opticaldevice as defined in claim 28, wherein the high confinement waveguide ofthe electro-optic element is optically coupled to the high confinementwaveguide of the passive optical element through a directional couplerbetween the high confinement waveguides.
 31. An integrated opticaldevice as defined in claim 28, wherein the electro-optic elementcomprises a lithium niobate waveguide device, and wherein the passiveoptical element comprises a silica-on-silicon waveguide device.
 32. Anintegrated optical device as defined in claim 28, wherein theelectro-optic element and the passive optical element are opticallycoupled as a flip-chip assembly, where the high confinement waveguidesare formed on a top surface of the electro-optic element and on a topsurface of the passive optical element, and one of the electro-opticelement and the passive optical element is inverted to physicallycontact the high confinement waveguides together.
 33. An integratedoptical device as defined in claim 32 further including an indexmatching optical glue disposed between the high confinement waveguide ofthe electro-optic element and the high confinement waveguide of thepassive optical element.